Multi-layer films made from polyamides and polyolefines have numerous advantageous characteristics such as great toughness, sealability and low permeability to water vapour, oxygen, carbon dioxide, fats and aromatic substances. They are therefore a preferred means for packaging foodstuffs, not least because of their high transparency and the fact that they can be printed on, whereas many of the characteristics are not present in single-layer films made from polyamides or polyethylene. It stands to reason that the films must also be easy to manufacture and simple to process.
Composite films can be produced by coating, laminating or coextrusion. Copolyamides are polyamide components particularly suitable for blow-coextrusion since homopolyamides do not yield adequately transparent films. With films made from partly crystalline polyamides, on the other hand, the gas permeability increases as the crystallinity decreases, as a result of which the barrier characteristics of films made from copolyamides are less good owing to the fact that their crystallinity is lower. For instance, as measured on a Polyamide 6 film with a thickness of 25 .mu.m, the oxygen permeation value amounts to 45 or 60 cm.sup.3 .multidot.25 .mu.m/m.sup.2 .multidot.d.multidot.bar respectively. A copolyamide made from Polyamide 6 and about 6% polyamide units of isophorone diamine and isophthalic acid has a permeation value of 60 or 75 cm.sup.3 .multidot.25 .mu.m/m.sup.2 d.multidot.bar (dry climate or moist climate, respectively).
Entirely amorphous polyamides of isophthalic acid and hexamethylene diamine or copolyamides with terephthalic acid have, in a dry climate, similar oxygen permeation values as Polyamide 6, whereas the steep increase in the permeation values of moist gases, as observed with Polyamide 6, does not occur with these amorphous polyamides. Films made from amorphous polyamides have optically attractive characteristics such as high transparency and gloss, but owing to their high modulus of elasticity they are too stiff to serve as packaging for foodstuffs, do not, in vacuum packages, adapt themselves so smoothly to the contours of the contents and are inclined to develop crease fractures or cracks. That is why they are preferably used to manufacture hollow bodies by the blow-coextrustion process.
It is known from DE 1 669 476 how to produce copolyamides from .epsilon.-caprolactam, hexamethylene diamine and isophthalic acid. The use of such copolyamides consisting of 70 to 95 wt.-% of .epsilon.-caprolactam and 5 to 30 wt.- % of hexamethylene diamine isophthalate and/or hexamethylene diamine terephthalate in order to produce films capable of shrinking after biaxial stretching is described in the Japanese patent specifications 62 227-626 and 63 214-445. Not all of these products have the required combination of transparency, gloss, gas permeability and processing characteristics.